Vitamin K Antagonist Anticoagulant Pregnancy: Contraindicated in the first and third trimesters (warfarin embryopathy / 'foetal warfarin syndrome'; risk of fetal bleeding and stillbirth). Can be used during breast-feeding.

Warfarin

Brand names: Coumadin, Marevan

Warfarin is an oral vitamin K antagonist anticoagulant used for conditions such as atrial fibrillation, venous thromboembolism and mechanical heart valves; this page focuses on its use in older people.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: Induction 10 mg once daily for 2 days; maintenance usually 3–9 mg once daily

Route: Oral

Frequency: Once daily, at the same time each day

Elderly: as for adults, but the dose may need to be lowered. Age ≥65 is a bleeding risk factor and skin necrosis with loading doses over 10 mg occurs mainly in obese elderly women (usually within 3–5 days of starting) — consider avoiding a high loading dose. Induction dose should be tailored to individual requirements; maintenance depends on the prothrombin time / INR and is adjusted to coagulation-test results. Make control (INR) tests at regular intervals, monitoring more frequently in those at higher risk of over-anticoagulation. In emergencies, initiate anticoagulation with heparin and warfarin together. Paediatric population: no data available.

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC; US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

Hypersensitivity to warfarin or any excipient
Haemorrhagic stroke
Clinically significant bleeding
Within 72 hours of major surgery with risk of severe bleeding
Within 48 hours postpartum
Pregnancy (first and third trimesters)
Concomitant drugs where interactions may significantly increase the risk of bleeding
Any condition in which the risk of haemorrhage outweighs the clinical benefit of anticoagulation

Side effects

Haemorrhage (including gastrointestinal, cerebral, epistaxis, haematuria)
Skin necrosis and 'purple toes' syndrome
Alopecia, rash, purpura
Nausea, vomiting, diarrhoea
Jaundice / hepatic dysfunction
Hypersensitivity

Interactions

Miconazole — enhances anticoagulant effect; monitor and titrate carefully
NSAIDs — increased risk of bleeding
Antiplatelet drugs — increased risk of bleeding

Clinical monograph

How it works

It inhibits vitamin K epoxide reductase, reducing synthesis of vitamin K-dependent clotting factors II, VII, IX and X and the anticoagulant proteins C and S.

Prescribing in practice

Older people have a higher bleeding risk and greater sensitivity, so initiate at lower doses with careful INR monitoring; assess bleeding versus thrombotic risk and consider falls and cognitive ability to manage therapy.
It has a narrow therapeutic range and numerous drug, food and alcohol interactions (including many antibiotics and changes in dietary vitamin K), all of which can destabilise the INR.
It is contraindicated in pregnancy and requires prompt action with bridging or reversal around bleeding, surgery or supratherapeutic INRs.

Monitoring

Monitor the INR regularly against the target range, more frequently after starting, dose changes or any interacting medicine, especially in older patients.

Counselling the patient

Attend all INR appointments and keep dietary vitamin K intake consistent.
Report unusual bleeding or bruising, and tell any clinician you take an anticoagulant.
Carry an anticoagulant alert card and check before starting new medicines.

Evidence & guidelines

Trials such as BAFTA showed warfarin reduces stroke in older patients with atrial fibrillation, and NICE supports anticoagulation with careful monitoring in this group.

Reference: NICE NG196 (AF); ESC AF Guidelines 2020; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Calculators

Pathways